Development of effective and safe vaccines against human immunodeficiency virus (HIV) infection is an urgent need to prevent the global dissemination of the virus. Some acquired immunodeficiency syndrome (AIDS) vaccine strategies have been evaluated in primate lentivirus infection models, but none of them has consistently induced sufficient resistance against the disease (Almond, N. M. and Heeney, J. L., 1998, AIDS 12 (Suppl. A): S133-140; Burton, D. R. and Moore, J. P., 1998, Nat. Med. 4: 495-498; Hulskotte, E. Q. et al., 1998, Vaccine 16: 904-915; Letvin, N. L., 1998, Science 280: 1875-1880).
Live viral vector-based vaccine is one of the promising prophylactic strategies because of its ability to induce efficient and durable antigen-expression (Cairns, J. S. and Sarver, N., 1998, AIDS Res. Hum. Retroviruses 14: 1501-1508; Hiresch, V. M. et al., 1996, J. Virol. 70: 3741-3752; Buge, S. L. et al., 1997, J. Virol. 71: 8531-8541). Poxvirus vectors have most often been used. There are a number of other options including adenoviruses. However, the attempts using these vectors have failed to elicit sufficient protective immunity against HIV. The efficiency to elicit protective immunity would be dependent on many factors such as the level and the duration of antigen-expression, the kinetics of the vector virus replication, and the tropism and the pathogenicity of the vector virus. Each viral vector currently available has both merit in some aspects and demerit in others. Precise evaluation and comparison of viral vectors would be required to find the optimal one.
One critical demerit of viral vector-based vaccine strategies is the induction of vigorous immune responses against the vector virus-derived antigens rather than the target antigens. This problem can be solved by using two or more different kinds of viral vectors for priming and boosting, respectively. DNA vaccine-based priming followed by viral vector-based boosting is also a favorable strategy (Hanke, T. et al. 1999, J. Virol. 73: 7524-7532; Robinson, H. L. et al., 1999, Nat. Med. 5: 526-534). Thus, development of a novel class of viral vectors is still waited for.